1. Field of the Art
The present invention relates generally to a material regression sensor and more particularly to an electrically resistive surface regression and ablation sensor for detecting and continuously measuring the instantaneous surface regression in materials. The invention has particular applicability for measuring instantaneous surface regression in hybrid rocket solid fuels and solid oxidizers, solid rocket propellants, ablative rocket nozzles, thermal protection materials, and other subliming, melting, wearing, or ablating materials. The invention also relates to a method of measuring such instantaneous surface regression. Regression in this case is defined as the dimensional loss of material from the surface of objects in question.
2. Description of the Prior Art
A long-standing need exists in the rocket propulsion community for an accurate, low cost, and reliable means of measuring surface regression in solid propellants, hybrid rocket solid fuel and solid oxidizer grains, ablative nozzles, thermal protection materials, and other materials. Two publications describe work by others to address this need.
A first publication entitled An Instrument for Real Time Measurement of Solid Rocket Motor Insulation Erosion was published at the 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit on Jun. 20-24, 1999. This publication discloses a pair of twisted wires which are polyimide insulated. The twisted wires are mounted in the motor insulation in a position in which the tip of the wires will recede with the erosion of the rocket motor internal insulation. As the combustion proceeds, the tips of the wires are melted to provide a completed circuit. A constant current applied along the wires will detect the resistance change via a voltage drop across the wires as the wire pair recedes with the decomposition of the insulation. This sensor is designed for a specific type of material and relies on the presence of a combustion process to melt the wires and thereby complete the electrical circuit. A problem with this particular sensor is that it may tend to disrupt or otherwise adversely effect the burning of hybrid fuels and solid propellants, thereby resulting in an uneven combustion surface. Another issue with this concept is that the location of the twisted wire tip within the host material is dependent upon the host material properties. A further issue with this concept is that it has been shown to be unreliable in certain materials.
A second publication entitled Scope of Capacitive Methods in Solid Propellant Diagnostics was published in the Journal of Propulsion and Power, Vol. 15, No. 6, November-December 1999. This publication disclosed an apparatus for instantaneous regression rate measurement of solid rocket propellants. The sensor of this publication is based on measuring capacitance, not resistance. Further, the concept of this sensor uses the host material, the propellant whose regression is being measured, as an active portion of the sensor. This adversely effects the reliability of the sensor and limits its applicability to unmetalized propellants. Additionally, this concept requires accurate knowledge of the solid propellant capacitance, which may vary from batch to batch, and thus, require calibration for each propellant batch. A further limitation of this concept is that substantial analytical correction factors must be applied to the results to account for electrical noise derived from combustion of the solid propellant.
Approaches utilizing ultrasonic and X-ray technologies have also been considered for measuring surface regression. Ultrasonic systems emit a pulse of sound at one side of the host material and measure the time it takes for the pulse of sound to travel through the material, or alternatively, the time it takes for the pulse to reflect off of the other surface of the material and return to the point of origin. Correlating this travel time with the speed of sound in the host material allows a calculation of the instantaneous material thickness. X-ray systems function by passing X-rays through the host material and measuring the attenuated X-ray intensity on the other side. The spatial intensity of the signal can be reduced to arrive at the material thickness as a function of time. Both ultrasonic and X-ray systems can provide accurate regression data, however, they require exceedingly high investments in personnel training and hardware, and also require in-situ calibration. Additionally, both X-ray and ultrasonic systems are not well suited for use onboard flight systems due to their relatively large size, high mass, and high power requirements.
Accordingly, there is a need in the art for a low-cost material regression sensor and more particularly to a material regression sensor which can accurately, reliably, continuously, and instantaneously measure the regression rate of materials, and in particular, of solid fuel or oxidizer hybrid rocket grains, solid rocket propellants, and other materials.
The present invention relates to a resistive regression and ablation sensor which can accurately, reliably and affordably measure surface regression, and acquire the data as a function of time. The sensor of the present invention can accurately measure the time dependent regression, ablation, or wear rate of a material, and thus eliminate the need to conduct multiple tests or make multiple measurements to determine the instantaneous dimensions of the material being monitored. Additionally, the sensor does not require any end user calibration or personnel training.
In general, the sensor of the present invention includes an electrically conductive, but high resistivity, sensor layer applied to a substrate and a means for applying a current (or voltage) to such sensor layer and measuring the voltage drop (or current) across the layer. Preferably, the sensor layer is embedded within the test material whose regression is desired to be measured with a first end at or near the regressing surface, and perpendicular thereto, and a second end connected with a pair of measurement leads. The sensor layer is embedded in the material such that as the material regresses, the length of the sensor shortens. Because the resistance of the sensor layer is related to its length, the amount which the material surface regresses can be instantaneously and continuously measured by applying a current (or voltage) to the measurement leads and measuring the voltage drop (or current) across the sensor layer.
More specifically, the surface regression sensor of the present invention includes an elongated, non-conductive substrate and first and second electrically conductive legs applied to the substrate and extending from a measurement end to a free end of the sensor. The legs are spaced from one another and are preferably parallel to one another. The electrically conductive, high resistivity sensor layer is applied to the substrate between the conductive legs and is electrically connected to the legs. Preferably, the conductive legs are of a relatively low resistivity material, while the sensor layer is of a relatively high resistivity material. The sensor layer and conductive legs in the preferred embodiment are a thin film, conductive ink, or other electrically conductive material.
First and second measurement leads are electrically connected with measurement ends of the conductive legs, and the sensor is embedded in the material whose regression is to be measured by positioning the free end of the sensor at the regression surface. By applying a current (or voltage) to the measurement leads and measuring the voltage drop (or current) across the sensor layer, the instantaneous thickness and regression rate of the test material can be determined.
The method aspect of the present invention relates to a method of measuring the regression of a material having a regressing surface. The phenomena causing the regression of the surface may include, but is not limited to, abrasive or other wearing, melting, ablation, sublimation, and combustion. Specifically, the method includes providing a sensor with a pair of spaced electrically conductive legs, an electrically conductive sensor layer extending between the legs and a pair of measurement leads electrically connected to the pair of legs. The method further includes embedding the sensor in the material whose regression measurement is desired with one end of the legs extending to the regression surface. Alternatively, the sensor can be installed behind the regression surface but will not detect or begin regression measurement until the surface has regressed to the test end of the sensor. Finally, the method includes applying a measurement current (or voltage) to the measurement leads and measuring the voltage drop (or current) and thus the resistance across the legs and the conductive sensor layer and from that information determining the extent of material regression.
Accordingly, it is an object of the present invention to provide a material regression sensor which can accurately and reliably detect and measure surface regression in a material.
Another object of the present invention is to provide a sensor for continuously measuring the instantaneous regression of a hybrid rocket fuel grain, solid rocket propellant, ablative, or other material.
A further object of the present invention is to provide a miniature resistive regression sensor for a hybrid rocket fuel grain, solid rocket propellant, ablative, or other material.
A still further object of the present invention is to provide a method for continuously and instantaneously measuring the regression of a hybrid rocket fuel grain, solid rocket propellant, ablative, or other material.
These and other objects of the present invention will become apparent with reference to the drawings, the description of the preferred embodiment and the appended claims.